Circuit breakers, line switches, disconnect switches and capacitor switches are well known components of electric transmission and distribution systems. Within these devices, spring-driven acceleration mechanisms have been used to accelerate penetrating contactors to sufficient velocity to extinguish an arcing contact occurring across a contactor gap within the switch without experiencing an undesirable restrike, which could otherwise cause disturbances on the electric power system. This typically requires extinguishing the arc after one-half cycle, which prevents a restrike from occurring after the initial arc break that occurs at the first half-cycle zero voltage crossing after initial separation of the contacts. For this type of device, it is helpful to house the penetrating contactor within a sealed container filled with a dielectric gas such as sulphur hexafluoride (SF6), which is directed into the contactor gap by a nozzle to help extinguish the arc. Extinguishing the arc in this manner, which is specifically designed to effectively absorb the arc energy, reduces the contactor gap separation required to extinguish the arc from what would be required to extinguish the arc in another environment such as air.
The basic design challenge for this type of device involves engineering an acceleration mechanism that obtains the desired contractor velocity quickly enough to extinguish the arc without experiencing an undesired restrike. An example of this type of device is shown in Rostron et al., U.S. Pat. No. 6,583,978 entitled “Limited Restrike Electric Power Circuit Interrupter Suitable For Use as a Line Capacitor and Load Switch,” which is incorporated herein by reference. In addition, other types of spring-driven acceleration mechanism have been used to accelerate penetrating contactors for many years. In general, spring-driven acceleration and toggle mechanisms for accelerating penetrating contactors for single- and three-phase electric power switch configurations are well known.
Using this type of device as a capacitor switch imposes an added objective of breaking the arc occurring across the contactor gap without experiencing an undesirable switching surge caused by the inrush current into the initially discharged capacitor. As is well known in the electric utility industry, the inrush current into the initially discharged capacitor spikes when the switch closes because the capacitor initially behaves like a theoretical short circuit. In typical electric power applications, this transient inrush current can spike to three or more times the rated current of the electric power circuit. The resulting current transient also causes a transient surge in the voltage of the electric power system. For example, voltage surges in the power system of 1.7 per-unit (i.e., 1.7 times the operating voltage) have been caused by capacitor switching in typical electric power applications.
One option for constructing a capacitor switch that reduces these types of system disturbances is to introduce a charging impedance into the circuit just prior to closing the power contactor that introduces the capacitor into the power circuit. The charging impedance typically includes a resistor, an inductor, or a combination of a resistor and an inductor. This approach initially charges the capacitor through the charging impedance, which prevents the inrush current from spiking when the initially discharged capacitor is first introduced into the circuit. Reducing the capacitor inrush current with a properly sized charging impedance allows reduces the voltage surge and associated voltage disturbance occurring on the electric power system. For this type of capacitor switch, the impedance contactor, as well as the charging impedance itself (or impedances), can be located inside or outside the container that houses the main power contactors.
For example, Leeds, U.S. Pat. No. 3,538,276, which is incorporated herein by reference, shows a circuit breaker with impedances located on the interior of the container filled with dielectric gas. These impedances are entered into the circuit prior to the closing of the main power contacts on switch's closing stroke. However, this device requires a large container to house the charging impedances. In addition, the switching device described in this patent includes a rotary contactor acceleration device that is cumbersome and requires a much larger container than a linear moving contactor arrangement. Therefore, this design is appropriate for a high voltage circuit breaker, but it is an expensive and relatively unreliable alternative for use as a capacitor switch that is intended to operate daily or several times a day.
Capacitor switches with external charging impedances and external impedance contactors have also been developed. These devices have been designed to close the impedance contactor before the main power contactor on the closing stroke, and to open the impedance contactor prior to the main power contactor on the opening stroke, as is desirable for a capacitor switch. However, these devices have conventionally relied an external charging impedance (or impedances) introduced into the circuit through an external whip. See, for example, Anand et al., U.S. Pat. No. 6,597,549 entitled “Capacitor Switch With External Impedance and Insertion Whip,” which is incorporated herein by reference. Although this device avoids the large container of the Leeds circuit breaker and implements the contactor closing sequence on the opening and closing strokes desired for a capacitor switch, the external whip is exposed to the weather elements. As a result, the whip can become frozen in place during freezing rain or sleet condition, which can disable the whip portion of the device and thereby decrease its reliability. For this reason, the external whip design alternative is most suitable to climates that do not experience a significant amount of frozen precipitation. In addition, the external moving components of the whip configuration increase the cost and complexity of the device, and can impose a significant additional maintenance requirement for the capacitor switch, which is intended to operate daily or several times a day for most application.
Accordingly, there is an ongoing need for a cost effective electric power switch suitable for use as a capacitor switch. There is a further need for a capacitor switch that includes a charging impedance that does not rely on an unduly large container filled with dielectric gas or an external insertion whip.